The nature and severity of hearing loss among hearing impaired individuals varies widely. Some individuals with linear impairments, such as that resulting from conductive hearing loss, can benefit from the linear amplification provided by conventional hearing aids using analog signal processing. Such aids may have the capacity for limited spectral shaping of the amplified signal using fixed low pass or high pass filters to compensate for broad classes of spectrally related hearing deficits. However, many types of hearing loss, particularly those resulting from inner ear problems, can result in non-linear changes in an individual's auditory system. Individuals who suffer such problems may experience limited dynamic range such that the difference between the threshold hearing level and the discomfort level is relatively small. Individuals with loudness recruitment perceive a relatively small change in the intensity of sound above threshold as a relatively large change in the apparent loudness of the signal. In addition, the hearing loss of such individuals at some frequencies may be much greater than the loss at other frequencies and the spectral characteristics of this type of hearing loss can differ significantly from individual to individual.
Conventional hearing aids which provide pure linear amplification inevitably amplify the ambient noise as well as the desired signal, such as speech or music, and thus do not improve the signal to noise ratio. The amplification may worsen the signal to noise ratio where an individual's hearing has limited dynamic range because the noise will be amplified above the threshold level while the desired speech signal may have to be clipped or compressed to keep the signal within the most comfortable hearing range of the individual.
Although hearing impaired individuals often have unique and widely varying hearing problems, present hearing aids are limited in their ability to match the characteristics of the aid to the hearing deficit of the individual. Moreover, even if an aid is relatively well matched to an individual's hearing deficit under certain conditions, such as a low noise environment where speech is the desired signal, the aid may perform poorly in other environments such as one in which there is high ambient noise level or relatively high signal intensity level. The limitations of conventional analog hearing aids can be overcome in hearing aids which employ digital signal processing, such as disclosed in copending application Ser. No. 07/120,286 entitled Adaptive Programmable Signal Processing Hearing Aid, filed Nov. 12, 1987.
Feedback is a common and annoying phenomenon in hearing aids. The wearer usually hears it as a loud, high frequency squeal when he moves close to a sound-reflecting surface, such as a wall, or when he turns up the volume knob to a high setting. The feedback instability in a hearing aid has the same causes as feedback in a public address system. A sound comes into the microphone of the hearing aid, is amplified and sent out by the receiver. It then leaks back to the microphone and starts around the loop again. If the loop gain of the system (hearing aid plus acoustic feedback path) is greater than or equal to unity and the phase is a multiple of 360.degree. at any frequency, then the output at that frequency will quickly rise in amplitude until it reaches the maximum output level for the aid.
The simplest way to stop feedback in an aid is to reduce the gain of the aid. If it is reduced sufficiently so that the gain around the loop is less than unity at all frequencies there will be no feedback instability. But the main purpose of hearing aids is to provide gain, so reducing the gain may not be a good solution for many people who have moderate to severe hearing losses. The other way to prevent feedback is to reduce the gain in the acoustic feedback path, the path between the receiver and the microphone. This can be done physically by using a tight fitting ear-mold without a vent-hole. The disadvantage is that tight fitting ear-molds can be uncomfortable and the use of a vent-hole may be necessary to give a desired frequency shaping.
Feedback in a digital hearing aid arises from exactly the same causes as in a conventional analog aid. The only slight difference is that digital aids tend to introduce a small delay to the signal. An analog aid will have a delay of the order of 100 microseconds whereas a digital aid may have a delay of perhaps 5 milliseconds. This causes no perceptual problems and it does not change whether or not feedback occurs, but it will change the rate at which instability climbs to the maximum level. Longer delays will cause a slower climb, although it may still be perceptually fairly fast.